This invention pertains to the field of control systems for scale model railroad layouts, and specifically to improvements in elements of block occupancy and location detection methods that are employed on model railroads.
Improvements in the miniaturization, increased capability and decreasing costs of electronics components coupled with new circuit designs have allowed the application of new techniques to model railroad layouts. Those advances permit the creation of layouts with greater levels of sophistication, automation and real time feedback of operating states from many types of devices on or around the layout.
Track occupancy detection for model railroads has ben used for many years. It is used for both operation of signal systems and also to display track state for areas out of direct view of the engineer or controlling dispatcher. Most practical and commercial products employ derivatives of the 1958 era Westcott "Twin T" circuit that uses back to back or bilaterally connected semiconductor diodes to develop a detection voltage when current flows through in either direction. This permits reliable detection of rolling stock that draw power for motor or other loads or have detector resistors fitted to their wheel sets. Other methods such as that of Richley, U.S. Pat. No. 5,752,677, may operate without DC power consumption and have been suggested for performing occupancy detection for model railroads. These high frequency methods are analogous to some methods used by the real prototype railroads such as the method of Stillwell in U.S. Pat. No. 5,417,388.
In the model railroad case the metal rails are used for conducting power and locomotive control signals from the track power booster to the layout and powered rolling stock. There are two different methods employed for wiring model railroad layout when using modern Digital Command Control signals driven by track power boosters. These are "Direct Home" wiring and "Common Rail" wiring. The Common rail wiring is a direct descendant of earlier common rail wired DC or AC system method and employs a two-wire approach. Today the "Direct Home" architecture is being adopted more often because it enforces a more disciplined modular wiring strategy for the layout. It also benefits model railroad wiring by allowing a single type of wiring method from a booster to any track section, irrespective of whether the track section is a "reversing section" or not. The Direct home strategy employs an implied three-wire connection to the boosters. Here the safety-ground bonding conductor is separate from the track current carrying conductors.
Employing the common "Twin T" circuit arrangement for the Direct home strategy requires careful design to ensure an optimal design solution, and hence differs from a Common rail design. It is typically difficult to use a Common Rail detection system on a Direct Home wired layout without carefully arranging the detector power supplies and detection blocks to be in a single booster power district. Additionally, when using Digital Command Control signals, the capacitive loading of an unoccupied track section tends to falsely trip the simple "Twin T" detector strategy when the track detection block is large and has long feeder conductors with significant parasitic or stray capacitance to ground.
Signaling based on block occupancy detection allows the introduction of Automatic Block Signals, ABS, or Centralized Traffic Control, CTC, or other traffic control strategies to model railroad layouts. In addition to allowing operation in the exact same manner as the prototype railroads, the model layout has another useful possibility of employing computer directed and generated traffic for both automated operation or semi-automatic operation. This is valuable since on many of the larger and more complex model layouts it is infrequent that a full roster of trained operators is available at all times, unlike the prototype railroads that arc staffed 24 hrs a day for critical train movements. Thus the option of some form of computer assistance allows a greater level of realism and activity for the model railroader.
Key to employing computer automation is a method of detecting both block occupancy of a track section and also detecting and identifying the rolling stock that is actually in the block. This ensures that the computer program does not need to consider an infinite set of possible layout states, error conditions or inferred locations of rolling stock, since it can monitor the exact state of the layout at any time. Notably, operators tend to move locomotives and rolling stock around the layout after derailments or coupling breaks or other actions, in a manner that the real railroads cannot do. The model railroader can simply pick up and move rolling stock from one location to another, creating havoc with a system that can't make a positive identification of rolling stock and its location. Practical computer enhancements need positive identification of rolling stock and its location. An alarm to indicate the addition or removal of equipment and the location of the action is a very useful detection improvement.
The capability of addressing or interrogating a particular device on the layout, detecting a predetermined coded response and then being able to determine its location is termed transponding. As for track occupancy detection, it is most common to use current conducted via the tracks to perform transponder detection. It is possible to perform the identification function with for example; Radio Frequency Identification techniques, infrared emitters, acoustic emitters and even bar codes or color coded areas detected by an optical scanner. Feedback by current is preferred since a continuous metallic circuit is conveniently available with the tracks running throughout the layout.
The acknowledgement pulses generated by a particular transponder device are defined to occur directly after, and to be time synchronized to, commands that a transponder recognizes arc addressed to its attention. These pulse responses are then an "identification acknowledgement" that is prompted by the system. This directly links the detection of valid current pulses to the address of the command that has just been sent and thus allows the address of the responding transponder to be inferred. By having a number of independent transponder detectors monitoring different track sections is possible to both determine the address of a transponder on the layout and also to localize its location to a specific track section.
Zimo Electronics has commercially demonstrated pulsed current unit identification of mobile locomotives on digitally controlled and powered layouts in Austria. The method used is the generation of brief but large acknowledgement or feedback current pulses at predetermined time windows by the controlling unit, or decoder, in the locomotive. The method uses four individual current pulses for a single acknowledgement, or ACK, and these are grouped as two pulse pairs in alternate voltage cycles. The large magnitude of these current pulses, typically larger than the motor operating currents, allow for pulse detection in the presence of additional current draws of motors lights and other power usage on the layout.
This implementation of transponder technology suffers from several technical limitations. Allowing the motor driver electronics to create a brief short circuit across the applied track power generates the Zimo current feedback pulse. This allows large and detectable current pulses. It does not provide an inherently safe or well-controlled or defined maximum current, typically needed for long term reliability. As transponders or decoders are made smaller, it becomes problematic to equip then with electronics robust enough to provide for these uncontrolled high current pulses, particularly when available track currents arc being increased to allow more concurrent locomotive operations on the same track section. The large currents created by this short circuit method also lead to potential radio interference problems, since the layout and tracks are unshielded and can radiate. The ACK current pulses used cause fast changing voltage fluctuations that increase radiation as the number of active transponders increase. Meeting the statutory and legal requirements around the world for interference suppression becomes burdensome with this method. The repeating high current spikes may interfere with or defeat the power management and short circuit protection logic of boosters or other power controlling devices.